1. Industrial Field of the Invention
This invention relates to modified steel slag or iron slag and a method of manufacturing the same, the modified steel slag being slag which is obtained by modifying furnace slag such as iron slag or steel slag, and, more particularly to modified steel slag or iron slag and a method of manufacturing the same, the steel slag being effective for preventing pulverization and generation of yellowish turbid water at the time of contact with water in iron slag or steel slag such as blast furnace slag.
2. Prior Art
A type of steel slag, particularly stainless steel slag, which has a basicity (weight ratio CaO/Si.sub.2) of substantially 1.5 or more has a property whereby the 2CaO SiO.sub.2 phase is changed from .alpha.-type phase to .alpha.'-type phase, and then changed to .gamma.-type phase or .beta.-type phase when the slag is subjected to a cooling-down process. In many cases, when the slag changes from the .alpha.'-type phase to the .gamma.-type phase, a volume expansion of substantially 14% results. As is well known, this causes the slag to pulverize into fine particles and thus become dust.
This type of degradation phenomenon worsens the working environment, and disturbs further utilization of slag. These present stainless steel manufacturers with serious problems in regard to the treatment of slag.
It has for a long time been a problem for stainless steel manufacturers to find a method of preventing the degradation of slag and of solidifying it since the discharged slag can be effectively utilized as a secondary material in such applications as civil aggregates for road construction and so on.
Known methods of restricting the pulverization of slag can be exemplified as follows:
.circle.1 . a method in which slag is made into water-granulated glass when the residue slag is discharged;
.circle.2 . a method in which slag is modified to form a material which mainly comprises CaO.SiO.sub. 2 and has a basicity of 1.5 or less (in practice this can be slightly varied due to the composition of the slag);
.circle.3 . a method in which the phase change from .alpha.'-type phase to .gamma.-type which results in a great change in density is restricted and the phase change from .alpha.'-type phase to .beta.-type phase is activated.
However, in regard to .circle.1 , at the time of water granulation, phreatic explosion can occur due to the presence of molten metals carried at the time of discharge of the slag, and since water granulated glass is a soft material, it has insufficient strength to serve as a construction aggregate. Therefore, this method .circle.1 has not yet been put into practical use although it has been partially tested.
In regard to .circle.2 although some additives designed to modify the properties of SiO.sub.2 containing material have already been placed on the market, they require the installation of supplying facilities and stirring facilities since a large quantity of SiO.sub.2 needs to be employed equivalent to substantially 20% of the fused slag. Furthermore, the viscosity of slag is increased due to the drop in temperature of the fused residue following the addition, and this is not suitable from the viewpoint of workability and total cost.
The method .circle.3 , that is, bringing about a phase change from .alpha.'-type phase to .beta.-type phase has been studied for many years and a variety of methods have been disclosed. One of these methods, which is the most effective and assured method available at present, [see Japanese Patent Laid-Open No. 43690/1978 and the Kawatetsu Engineering Report Vol. 18, No. 1 (1986) 20 to 24 in which Si.sup.4+ ions are replaced by B.sup.3+ ions which have a smaller diameter than that of the Si.sup.4+ ions contained in the slag].
However, the above-described conventional boron type of slag pulverization preventing material is in the form of small powder and dehydration/vaporization reaction occurs at the time of contact with the fused slag since the boronic slag pulverization preventing material is a water containing material. As a result of this, a blowing phenomenon of the slag pulverization preventing material is generated, causing the working environment to become excessively worsened and danger sometimes involved in the work. Therefore operation of the work is very difficult.
Furthermore, since the conventional boron type of slag pulverization preventing material significantly differs in the chemical composition and property from slag, the difference in viscosity and density from the fused slag can be easily generated, that is, a so-called affinity between the slag and the slag pulverization preventing material is not sufficient and thereby the diffusing/mixing performance is insufficient. As a result of such disadvantages, the boron type of slag pulverization preventing material cannot be put into practical use although there have been some disclosures upon it.
A second problem arises in iron slag or steel slag such as blast furnace slag that generation of a so-called "yellowish turbid water" at the time of bringing slag into contact with water such as rain or gutter water.
As is well known, gradually-cooled down slag such as blast furnace slag is widely used as various aggregates, particularly as road beds, that is a so-called "ballast". However, it has been confirmed that if the percentage of sulfur contained in slag is high, a mistake in conditions for use and manner to use it will cause yellowish turbid water and smell of hydrogen sulfide to be generated due to water or gutter water which has been brought into contact with slag. Particularly in order to secure slag quality for road construction to be free from such problem, it is standardized that slag should not generate any yellowish turbid water and smell of hydrogen sulfide. In order to evaluate this fact, a color identification test is employed, and slag should satisfy this test (JIS A 5015 made public on November 1).
A phenomenon of generation of yellowish turbid water is caused from elution of sulfur (S) contained in the form of calcium sulfide (CaS) which is contained, as a major part, in slag, and is due to generation of yellow polysulfide (such as CaSx) after being applied to hydrolysis process. Known methods of preventing generation of yellowish turbid water can be exemplified as follows:
.circle.1 a method in which slag is subjected to aging in which it is oxidized by water and air so that it is stabilized;
.circle.2 a method in which oxidant is added to fused slag;
.circle.3 a method in which slag is treated with CO.sub.2 2 so that the surface of the slag is stabilized; and
.circle.4 a method in which the cooling-down speed of slag is raised.
In regard to .circle.1 aging treatment takes almost one to three months to be completed, causing a very wide space for storing to be provided.
In regard to .circle.2 several methods can be exemplified such as a method in which high degree of ferrous oxide is added or a method in which a gas containing oxygen, such as air, is added. However, this method is not preferable since a poisonous SO.sub.2 gas is generated due to the reaction. Furthermore, with this method, the generation of the yellowish turbid water cannot be sufficiently prevented.
In regard to .circle.3 , although the surface of slag can be stabilized, this method involves a disadvantage that fused sulfides can be again overflowed from a crushed surface when it is crushed at the time of changing pressure.
The method .circle.4 is a method in which glass is prepared by degrading, diffusing and rapidly cooling down fused slag so that contained sulfur component is prevented from being oozed out. However in regard to .circle.4 , it involves deterioration in strength and its necessity of granulating to a level below a specific size to form the glass will allow it to be used as a material for small aggregates, but it is very difficult to be used as rough aggregates.
Although another method has been disclosed in which iron, manganese or zinc is, as an effective component, added so that sulfides are fixed, this method has not been put into practical use due to its high cost and insufficient effect.
Therefore, at present, the method .circle.1 in which slag is subjected to aging is only the available method to prevent generation of slag yellowish turbid water.